Prior art heating devices such as electric ovens or electronic ranges generally have an operating section as shown in FIG. 1. In other words, the device is comprised of an operating panel 3 including a time switch 1 of the rotary type for setting the heating time and a temperature-setting knob 2 having a control section such as an oil thermostat or bimetal. Numeral 4 designates a body, and 5 a door. As can be seen, most of the prior art operating sections comprise analog input means including a setting section of the rotary or slide type. This is attributable to the user's sense of familiarity with analog quantities, the setting facility, the low cost of control parts, etc.
On the other hand, recent development in electronics has been remarkable. Especially, the advent of the microcomputer and large scale integrated circuits is expected to make possible complicated controls at low cost. For this reason, the necessity has suddenly arisen for providing the control section as a digital system. FIG. 2 shows an example of the electronic range with a temperature sensor having such a digital control system. The operating panel 3 is provided with ten numeral keys 6 for inputting heating time and temperature and a time key 8 and a temperature key 9 for declaring which of the time and temperature setting operations are to be set on the control section and shown on the indication section 7. In addition, various functional keys including a start key for commanding a heating start are provided as required. Now, in this configuration, the temperature and time setting operations are effected by the following procedures. First, the temperature key 9 is depressed, followed by operating the numeral key 6 for setting the heating temperature. At this time, this set temperature is indicated as, say, "210C" on the indication section 7. Next, the time key 8 is depressed, followed by the depression of the numeral key 6 whereby the heating time is set. At this time, the indication section 7 switches from the temperature indication to the time indication, so that the heating time such as "1000" (10 minutes) is indicated.
The example shown in FIG. 2 has three inconvenient points from the viewpoint of operability. The first point is that such an indication is not clear from the fact that one indication section is used for dual purposes by switching. In other words, it is impossible to know the time and temperature at the same time. This disadvantage may be obviated by providing two indication sections. However, it requires another indication section of four digits each having seven segments, so that a total of 28 segments must be handled, thereby leading to a bulky control section. Secondly, temperature setting is complicated in that it is required to search for predetermined numeral keys three times after the depression of the temperature key. This is troublesome and lacks a good sense of operability as compared with the device shown in FIG. 1 in which setting is completed by a single turn of the temperature setting knob 2. Thirdly, if the present temperature is indicated at the beginning of heating, the set temperature is unknown, while if the set temperature is indicated at the beginning of heating, the present temperature is not known, thus making it impossible to comprehend the heating process. This shortcoming is common to the example shown in FIG. 1. Namely, in the case of a timer, the remaining time is known by the decrement of the set time. In the case of temperature which increases gradually, however, there has not yet been found any convenient method by which both the set value and the present value are known by such a simple indication.